Highly ductile steel sheet and method of manufacturing the same

ABSTRACT

A steel sheet has a C content in the range of 0.04 to 0.25% by mass, a Si content in the range of 0.05 to 3% by mass, a Mn content in the range of 0.1 to 3% by mass, an Al content in the range of 0.01 to 2% by mass, a P content of 0.02% by mass or below and higher than 0% by mass, and a S content in the range of 0 to 0.005% by mass. The number of CaO.Al 2 O 3  inclusions containing CaO, Al 2 O 3 , SiO 2  and MgO, having a CaO content of 5% or above, a SiO 2  content of 0.1% or above and an Al 2 O 3  content of 60% or above, respectively, having major axes of 30 μm or above, and contained in 1 kg of the steel sheet is seventy or below. The steel sheet is highly ductile and has high bore-expandability. A steel slab manufacturing method manufactures a steel slab for forming the steel sheet.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a highly ductile cold-rolled steelsheet having high bore expandability and a method of manufacturing sucha highly ductile steel sheet.

2. Description of the Related Art

Steel sheets for forming body frames of automobiles are required to havehigh ductility represented by bore expandability. Known inclusionsadversely affecting the ductility of steel sheets are oxides, such asAl₂O₃, and sulfides, such as MnS. Reduction of the amount of thoseinclusions contained in steel sheets is important to provide steelsheets with high ductility.

Techniques for improving the ductility of steel sheets disclosed in JP-ANos. 2000-144230 and 2002-327239 are intended to reduce the number ofalumina clusters. A technique disclosed in JP-A No. 2000-1736 forms ahighly ductile high-tension steel sheet containing oxide inclusionshaving a predetermined chemical composition and particle sizes in therange of 1 to 50 μm, and containing at least one of TiO₂, CaO and REMoxide.

Basically, those prior art techniques add Ti as an essential componentto the steel sheet to improve the ductility of the steel sheet. However,hard TiN inclusions are produced in the steel sheet in some cases andthe hard TiN inclusions deteriorate the ductility of the steel sheet.

A cold-rolled steel sheet disclosed in, for example, JP-A No.2002-212674, in which Ti is not an essential component, contains Mg.Cracks formed in the edge of a punched hole formed in this prior artsteel sheet containing Mg are minute and uniform, which proves that thesteel sheet has improved bore expandability. Although those prior arttechniques are effective in improving the ductility of steel sheets tosome extent, the ductility of steel sheets produced by those prior arttechniques is not necessarily as high as ductility required in recentyears.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide ahighly ductile steel sheet having high bore expandability and a methodof manufacturing a highly ductile steel sheet.

A steel sheet in a first aspect of the present invention has a C contentin the range of 0.04 to 0.25% by mass, a Si content in the range of 0.05to 3% by mass, a Mn content in the range of 0.1 to 3% by mass, an Alcontent in the range of 0.01 to 2% by mass, a P content of 0.02% by massor below and higher than 0% by mass, and a S content in the range of 0to 0.005% by mass; wherein the number of CaO.Al₂O₃ inclusions containingCaO, Al₂O₃, SiO₂ and MgO, having a CaO content of 5% or above, a SiO₂content of 0.1% or above and an Al₂O₃ content of 60% or above,respectively having major axes of 30 μm or above, and contained in 1 kgof the steel sheet is seventy or below.

A steel slab manufacturing method in a second aspect of the presentinvention includes the steps of: producing CaO slag containing CaO,Al₂O₃, SiO₂, MgO, MnO and Fe, and having a CaO content of 45% or above,a total Fe content of 5% or below, and a MnO content of 3% or below on amolten steel decarburized in a decarburizing furnace and contained in aladle; desulfurizing the molten steel to reduce the S content of themolten steel to 0.005% or below by stirring the molten steel and the CaOslag by a gas stirring method; circulating the desulfurized molten steelin a vacuum circulation degassing apparatus for 10 min or longer; andforming a steel slab by casting the molten steel circulated in thevacuum circulation degassing apparatus.

The steel sheet of the present invention can be produced from the thusproduced steel slab.

The present invention reduces the S content of the molten steeleffectively to suppress the production of sulfide inclusions byeffectively desulfurizing the decarburized molten steel decarburized bythe decarburizing furnace by the CaO inclusions of a properly adjustedcomposition, and reduces oxide inclusions originating from the CaO slagand adversely affecting the ductility of the steel to the least possibleextent. A highly ductile steel sheet can be produced by processing thesteel slab produced by casting the thus produced molten steel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Various inclusions contained in a steel are principal factors thatdeteriorate the ductility of steel sheets produced by processing thesteel and cause cracks when the steel sheet is processed. The reductionof such inclusions will improve the ductility of the steel sheet. When amolten steel having a high S content is cast to produce a steel slab,MnS inclusions precipitate at a stage of the cast molten steel and theMnS precipitated in the steel affects adversely to the ductility of thesteel. Thus it is important to suppress the precipitation of MnS so thatthe ductility of the steel may not be reduced.

The inventors of the present invention found through studies that theprecipitation of MnS can be effectively suppressed by reducing the Scontent of the steel to a value in the range of 0 to 0.005% by mass.Hereinafter, content in “% by mass” will be simply indicated in “%”.From the viewpoint of improving ductility, it is preferable that the Scontent of the steel is 0.004% or below, more preferably, 0.003% orbelow.

To reduce the S content of the steel to 0.005% or below, the moltensteel needs to be desulfurized in a refining process by mixingdesulfurizing slag in the molten steel. The desulfurizing slag may bemixed in the molten steel by any suitable conventional mixing method.For example, the desulfurizing slag may be mixed in the molten steel bya stirring method that blows a gas through a refractory lance into themolten steel to stir the slag and the molten steel or by a stirringmethod that blows a gas through a plug on the bottom of a ladle into themolten steel to stir the molten steel and the slag.

Desulfurizing slag containing CaO is used generally in the industry.Suitable desulfurizing slag is, for example, CaO, CaO.CaF₂ slag,CaO.Al₂O₃ slag, CaO.Al₂O₃.CaF₂ slag, CaO.Al₂O₃.MgO slag andCaO.Al₂O₃.SiO₂ slag. These substances will be inclusively referred to asCaO slag. To form the desulfurizing slag, namely, the CaO slag, on thesurface of the molten steel, a CaO flux containing slags above is addedto the molten steel. The respective amounts of CaO, total Fe and MnOcontained in the slag need to be controlled properly. Suppose that thesum of the respective amounts of CaO, Al₂O₃, SiO₂, MgO, MnO and total Fe(in some cases, referred to as “T.Fe”) is 100%(CaO+Al₂O₃+SiO₂+MgO+MnO+total Fe=100%). Then, the slag needs to have aCaO content of 45% or above. Effective desulfurization of the moltensteel cannot be expected if the CaO content of the slag is less than45%. Preferably, the CaO content of the slag is 48% or above, morepreferably, 50% or above. The CaO content of the slag can be adjusted to45% or above by using a CaO flux having a CaO content of 50% or above.

The slag formed on the surface of the molten steel contained in theladle contains FeO and MnO produced in the molten steel during refiningin an oxidation refining furnace, such as a converter or an electricfurnace. The desulfurizing effect of the slag decreases if the slagcontains an excessive amount of T.Fe content and an excessive amount ofMnO. Therefore the respective amounts of T.Fe content and MnO containedin the slag must be properly adjusted. The respective amounts of T.Feand MnO must be 5.0% or below and 3% or below, respectively, whencontaining CaO+Al₂O₃+SiO₂+MgO+MnO+total Fe=100%. Preferably, the amountof T.Fe contained in the slag is 4% or below and the amount of MnOcontained in the slag is 2% or below. A recommended T.Fe content and arecommended MnO content of the slag are 3% or below and 1% or below,respectively. In order to reduce the slag to adjust the T.Fe content andthe MnO content of the slag to values in the foregoing ranges, slagproduced during refining in the oxidation refining furnace is removedbefore the desulfurizing flux is added to the molten steel or Al, Si andC are added to the molten steel or the slag or to either of the moltensteel and the slag.

The foregoing desulfurizing process eliminates MnS inclusions thatreduce the ductility of the steel sheet. However, inclusions originatingin the desulfurizing slag (hereinafter, referred to as “slaginclusions”) and produced in the molten steel when the molten steel andthe slag are stirred for ladle refining remain in the molten steel whenthe molten steel is cast to produce a steel slab. The slag inclusionscontained in the steel slab reduce the ductility of steel sheets formedby rolling the steel slab.

The slag inclusions are identified by the following condition. The ladleslag has a CaO content of 45% or above and a SiO₂ content of below 10%.When the ladle slag and the molten steel are mixed by stirring, theladle slag reacts with Al contained in the molten steel and changes intoa slag containing CaO, Al₂O₃, SiO₂ and MgO, and having a CaO content of5% or above and a SiO₂ content of 0.1% or above (when containingCaO+Al₂O₃+SiO₂+MgO =100%). Such a slag is regarded as slag inclusions.The present invention deals with CaO.Al₂O₃ inclusions. The CaO.Al₂O₃inclusions contain CaO, Al₂O₃, SiO₂ and MgO and have a CaO content of 5%or above and an Al₂O₃ content of 60% or above. Thus the followingdescription will be made on an assumption that the CaO.Al₂O₃ inclusionscontain CaO, Al₂O₃, SiO₂ and MgO, and has a CaO content of 5% or above,a SiO₂ content of 0.1% or above and an Al₂O₃ content of 60% or above.

The inventors of the present invention made studies of the effect of theCaO.Al₂O₃ inclusions contained in steel sheets on the ductility of thesteel sheets and found that the CaO.Al₂O₃ inclusions having a major axisof 30 μm or above affect the ductility of the steel sheets and thereduction of the number of the CaO.Al₂O₃ inclusions contained in 1 kg ofthe steel sheet to 70 or below, preferably, 60 or below, more preferably50 or below improves the ductility of the steel sheets remarkably.

A vacuum circulation degassing process (hereinafter, referred to as “RHprocess”) needs to be executed after the desulfurization process toreduce the amount of the slag inclusions produced by the desulfurizationprocess and contained in the steel by flotative separation. A vacuumcirculation degassing apparatus needs to continue the RH process at 10min or longer to remove the slag inclusions by flotative separation.Preferably, the duration of the RH process is 15 minor longer, morepreferably, 20 min or longer.

The present invention provides a highly ductile steel sheet of, forexample, a thickness in the range of about 1 to about 3 mm byhot-rolling or cold-rolling a steel slab produced by casting the moltensteel processed by the desulfurization process and the RH process. Thehighly ductile steel sheet can be produced from the foregoing steel slabregardless of the method and conditions of rolling. The chemicalcomposition of the steel needs to be properly adjusted. The ranges ofchemical component contents of the steel of the present invention otherthan S content are determined for the following reasons.

C Content: 0.04 to 0.25%

Although the smaller C content is desirable to improve the ductility,particularly, bore expandability of the steel sheet, the strength of thesteel sheet decreases significantly when the carbon content is less than0.04%. When the C content is higher than 0.25% and the Mn content is inthe range specified below, pearlite grains occur excessively, thelamellar intervals of the pearlite grains widens and the balance betweenstrength and local ductility is deteriorated. Therefore, the C contentof the steel sheet of the present invention must be in the range of 0.04to 0.25%. A preferable lower limit C content is on the order of 0.05%and a preferable upper limit C content is on the order of 0.22%.

Si Content: 0.05 to 3%

Si is a useful effective solid solution strengthening element. The Sicontent of the steel sheet needs to be 0.05% or above to make Siexercise the solid solution strengthening effect. The solid solutionstrengthening effect of Si saturates at a certain Si content and hencean excessively high Si content is economically disadvantageous. Anexcessively high Si content will enhance the hot brittleness of thesteel sheet. Thus the Si content must be 3% or below. A preferable lowerlimit of the Si content is 0.1%, more preferably, 0.2% or above. Apreferable upper limit of the Si content is 2.5%, more preferably, 2% orbelow.

Mn Content: 0.1 to 3%

Manganese contained in the steel sheet stabilizes the austenitic phase,facilitates formation of a hard phase during cooling and strengthens thesteel sheet. The hard phase necessary for strengthening the steel sheetcannot be formed if the Mn content is excessively low. Therefore, the Mncontent must be 0.1% or above. Banded structures develop and theductility decreases if the Mn content is excessively high. Therefore, anupper limit Mn content must be 3%. Preferably, a lower limit Mn contentis 0.2%, more preferably, 0.3% or above. Preferably, an upper limit Mncontent is 2.8%, more preferably, 2.5% or below.

Al Content: 0.01 to 2%

Aluminum is necessary for the deoxidation of the molten steel and iseffective in promoting desulfurization during ladle refining by loweringthe potential of oxygen in the molten steel. The Al content must be0.01% or above to make such an effect of Al effective. Preferably, theAl content is 0.015%, more preferably, 0.02% or above. The effect of theAl content saturates at a certain Al content and hence an excessivelyhigh Al content is economically disadvantageous. Therefore, the Alcontent must be 2% or below, preferably, 1.5% or below.

P Content: 0.02% or below and higher than 0%

Phosphorus strengthens the steel and, at the same time, embrittles thesteel and deteriorates the ductility of the steel. An upper limit Pcontent must be 0.02%, preferably, 0.015%. The effect of P increaseswith the increase of the P content. Preferably, the P content is 0.003%or above, more preferably, 0.005% or above to make the effect of Peffective.

The molten steel contains, in addition to the foregoing basic componentelements, Fe and unavoidable impurities, such as Cr, Cu and Sn. Whennecessary, the steel sheet may contain additional trace elements, suchas Mo, V and Nb, in contents in the range of about 0.01 to about 0.05%.

Examples of the steel sheet of the present invention will be described.However, the examples are intended to illustrate the invention and arenot to be construed to limit the scope of the invention. Naturally,various changes and variations may be made in the examples specificallydescribed herein without departing from the scope and spirit of theinvention.

EXAMPLES

Two hundred and fifty tons of a molten steel decarburized in a converterwas tapped from the converter into a ladle. Various alloy steels wereadded to the molten steel by a ladle refining apparatus to adjust thechemical composition of the molten steel to a predetermined chemicalcomposition and the molten steel was desulfurized. Table 1 shows therespective compositions of desulfurizing slag. Argon gas (Ar gas) wasblown through the lance into the molten steel to stir the molten steel.The ladle was carried to a vacuum circulation degassing apparatus afterthe completion of the composition adjustment and desulfurization. Themolten steel was circulated for the RH process by the vacuum circulationdegassing apparatus. RH process times, namely, times of duration of theRH process, are shown in Table 1. Molten steels in Examples 3 and 4 werenot processed by ladle refining and molten steels in Examples 5 and 6were not processed by the RH process. TABLE 1 Molten steel processingprocess RH Composition of desulfurizing slag for ladle refining SampleLadle RH processing (% by mass) No. refining process time (min) CaOAl₂O₃ SiO₂ MgO T. Fe MnO Total 1 ◯ ◯ 13 51.8 26.2 8.5 7.1 5.2 1.2 100 2◯ ◯ 25 46.1 29.5 8.2 8.1 2.5 5.6 100 3 ◯ ◯ 17 39.5 36.8 8.1 9.0 2.5 4.1100 4 X ◯ 15 — — — — — — 100 5 X ◯ 19 — — — — — — 100 6 ◯ X — 53.1 31.36.2 7.1 1.1 1.2 100 7 ◯ X — 53.7 31.8 5.8 6.5 0.8 1.4 100 8 ◯ ◯ 7 49.934.4 4.8 7.5 1.5 1.9 100 9 ◯ ◯ 11 46.7 33.7 6.2 9.7 2.1 1.6 100 10 ◯ ◯15 48.2 36.1 5.1 8.2 1.1 1.3 100 11 ◯ ◯ 18 53.2 37.1 4.2 3.6 0.9 1.0 10012 ◯ ◯ 23 57.5 31.0 3.4 5.5 1.2 1.4 100

Steel slabs were produced by casting the molten steels processed by theRH process by a continuous casting process. The steel slabs weresubjected to a hot rolling process to produce hot-rolled steel sheets ofa thickness in the range of 3 to 4 mm. The hot-rolled steel sheets weresubjected to a cold rolling process to produce cold-rolled steel sheetsof a thickness in the range of 2 to 3 mm.

Sample pieces for a bore expand test were sampled form the cold-rolledsteel sheets. The sample pieces were tested by the flowing test methodto evaluate their bore expandability. The number of CaO.Al₂O₃ inclusionscontained in the steel sheets was determined by the following countingmethod.

Bore-Expandability

The sample pieces were punched to form bores of 10 mm in diametertherein and the punched sample pieces were subjected to a bore expandtest. In the bore expand test, the bore of the sample piece was expandedby pressing a conical punch having a point angle of 60° into the bore,and the diameter of the thus expanded bore was measured when cracksextending through the thickness of the sample piece were formed in theedge of the bore. A limit bore expandability λ was calculated by usingan expression:λ(%)={(D ₁ /D ₀)/D ₀}×100where D₀ is the initial diameter of the bore and D₁ is the diameter ofthe expanded bore. Steel sheets having higher ductility have higherlimit bore expandability.

Number of CaO.Al₂O₃ Inclusions

Inclusions contained in the cold-rolled steel sheet were extracted by amethod disclosed in JP-A No. 2002-340885. The cold-rolled steel sheetwas heated and held at 1000° C. for 30 min and then the steel sheet wascooled rapidly for a solution heat treatment to eliminate carbides fromthe steel sheet. Eight sample pieces of 3.2 mm×100 mm×50 mm each havinga weight of 1 kg were cut out from the cold-rolled steel sheet. Eachsample piece was immersed in a ferrous chloride solution of pH 6.0 todissolve 1 kg of iron matrix by a constant-current electrolysis method.Then, the solution was filtered by a filter having pores of 27 μm indiameter to obtain a residue containing CaO inclusions. The number ofCaO.Al₂O₃ inclusions containing CaO, Al₂O₃, SiO₂ and MgO, having a CaOcontent of 5% or above, a SiO₂ content of 0.1% or above and an Al₂03content 60% or above, respectively having major axes of 30 μm or aboveand contained in the residue was counted by using a scanning electronmicroscope combined with a wavelength dispersive x-ray spectrometer(SEM•WDS).

Table 2 shows the counted numbers of CaO.Al₂O₃ inclusions and thechemical composition of the steel sheets. TABLE 2 Number of CaO.Al₂O₃Bore Sample Chemical composition of steel sheet (% by mass) inclusionsper 1 kg expandability No. C Si Mn P S Al of steel sheet (%) 1 0.04 1.052.17 0.006 0.006 0.025 37 71 2 0.14 0.98 2.10 0.011 0.008 0.014 22 77 30.11 1.22 2.46 0.009 0.007 0.038 32 81 4 0.08 1.45 2.25 0.010 0.0090.027 40 58 5 0.08 1.31 2.41 0.010 0.007 0.031 35 83 6 0.07 1.65 1.510.010 0.001 0.034 96 68 7 0.16 1.52 2.23 0.013 0.002 0.027 89 49 8 0.090.07 0.82 0.005 0.001 0.029 81 90 9 0.15 0.52 1.28 0.009 0.004 0.028 63113 10 0.06 0.73 1.57 0.012 0.003 0.047 46 126 11 0.17 1.35 2.16 0.0100.002 0.036 19 131 12 0.13 0.95 2.00 0.006 0.004 0.044 34 120

It is known from the measured data shown in Table 2 that Samples 9 to 12meet all the requirements of the present invention. The numbers of theCaO.Al₂O₃ inclusions contained in Samples 9 to 12 are small and theSamples 9 to 12 have high bore expandability.

Each of Samples 1 to 8 does not meet some of the requirements of thepresent invention and has low bore expandability. As regards Samples 1to 5, although processing time for processing the steel sheet by the RHprocess is within a range specified by the present invention and thenumbers of CaO.Al₂O₃ inclusions are within a range required by thepresent invention, the composition of the desulfurizing slag used byladle refining does not meet conditions specified by the presentinvention (Samples 1 to 3) or the molten steel was not processed byladle refining (Samples 4 and 5). Consequently, the steel sheets inSamples 1 to 8 have a large S content and a low ductility.

The composition of the desulfurizing slag for the ladle refining of themolten steels for forming the steel sheets in Samples 6 to 8 meetrequirements of the present invention and have S contents of 0.0055 orbelow, respectively. However, the molten steels for forming the steelsheets in Samples 6 and 7 are not processed by the RH process afterladle refining and the molten steels for forming the steel sheet inSample 8 were processed by the RH process for a time shorter than 10min. Therefore, the numbers of CaO.Al₂O₃ inclusions contained in 1 kg ofthe steel sheets in Samples 6 to 8 are greater than seventy anddeteriorates bore expandability.

Although the invention has been described in its preferred embodimentswith a certain degree of particularity, obviously many changes andvariations are possible therein. It is to be understood that the presentinvention may be practiced otherwise than as specifically describedherein without departing from the scope and spirit thereof.

1. A steel sheet having a C content in the range of 0.04 to 0.25% bymass, a Si content in the range of 0.05 to 3% by mass, a Mn content inthe range of 0.1 to 3% by mass, an Al content in the range of 0.01 to 2%by mass, a P content of 0.02% by mass or below and higher than 0% bymass, and a S content in the range of 0 to 0.005% by mass; wherein thenumber of CaO.Al₂O₃ inclusions containing CaO, Al₂O₃, SiO₂ and MgO,contain 5% or above of CaO, 0.1% or above of SiO₂ and 60% or above ofAl₂O₃ in 100% of oxides including CaO, Al₂O₃, SiO₂ and MgO and havingmajor axes of 30 μm or above, and contained in 1 kg of the steel sheetis seventy or below.
 2. The steel sheet according to claim 1, whereinthe S content is in the range of 0 to 0.003% by mass.
 3. A method ofmanufacturing a steel slab for forming the steel sheet according toclaim 1, said method comprising the steps of: producing CaO slagcontaining CaO, Al₂O₃, SiO₂, MgO, MnO and total Fe, and having a CaOcontent of 45% or above, a total Fe content of 5% or below, and a MnOcontent of 3% or below on a molten steel decarburized in a decarburizingfurnace and contained in a ladle by adding a CaO flux to the moltensteel; desulfurizing the molten steel to reduce the S content of themolten steel to 0.005% or below by stirring the molten steel and the CaOslag by a gas stirring method in the ladle; circulating the desulfurizedmolten steel contained in the ladle in a vacuum circulation degassingapparatus for 10 min or longer; and forming a steel slabby casting themolten steel circulated in the vacuum circulation degassing apparatus.4. The method according to claim 3, the CaO slag has a CaO content of50% by mass or above.
 5. The method according to claim 3, wherein theCaO slag has a MnO content of 2% by mass or blow and a total Fe contentof 3% by mass or below.